Teardrop aerodynamics... in reverse!
 

Cd started a thread about which car's shape is closest to a teardrop shape.
Looking around on the streets I see a lot of teardrop shaped cars, but all of them are driving backwards! The efficient teardrop shape is round in the front and tapered in the back, not the other way around, as seen in this thread and this video.
From what I can see, car would be much more aerodynamic if they were driven in reverse. Here's what I mean:
This is how close a 5-door Citroen C4 is to a teardrop:
http://i193.photobucket.com/albums/z...t1teardrop.jpg
And here is the same car going backwards:
http://i193.photobucket.com/albums/z...t2teardrop.jpg
Which is closer to a teardrop?

You're right, except that the windshield slopes down too fast

I've noticed that the older the car, the closer the windshield is to vertical, while the hood is horizontal. The new trend is to blend the windshield and hood into one long slope. This helps the airflow move over the windshield instead of smacking into it. I guess the long engine bay it there for safety reasons.

whoa, poland, i'm 1/2 polish, & i went over to poland a couple years ago, was awesome, anyways

intresting theory...but i think having the front hood/bumper split the air before it hits the rest of of the car helps...but i could be wrong

Big end first is better (assuming proper taper aft).

See: http://ecomodder.com/forum/showthrea...-end-3708.html

I'm going to assert that it's very likely cars continue to have hoods extending in front of the passenger compartment because sitting in the front of the vehicle when you crash into something at speed is probably bad for your health.

Putting the windshield way out in front of the driver is probably good for aero, but much more than that it keeps your head away from the hard, unforgiving glass when you come to an abrupt stop upon impact.

But the windshield can't be at the extreme front of the car... because then you can't access the engine to perform repairs. The engine can't be in the passenger compartment because of fumes from a poorly maintained one and because of noise/vibration (everyone wants a silent cabin). The vehicle can't have a luggage trunk in front because depending what you keep in the trunk, your set of golf clubs cound become skewers if you slam into a brick wall.

So while the shape of cars today may not be ideal from an aero perspective, I think they are as they are because they have to be, to an extent. We'll see better and better compromises/solutions as time goes by, always do ;)

^ Wonder if they still build full size vans this way?

http://d.yimg.com/kq/groups/14334890...ge.jpg?type=sn

This is the best example I can think of. I currently have the Pontiac version, and had one identical to this except different wheels.

Makes me wonder about something. If the teardrop is the best aero shape, would the reverse teardrop be the worst shape? Probably not really, a parachute would be worst, but I wonder how they would compare anyway.

I used to pull my 5x8 enclosed trailer with my 6000 wagon. It was both narrower and lower than the trailer. 3.1 engine. Pulling it was a breeze, literally. I could sail down the highway at 65 easily, could have done 70 if I wanted to.

Now with the van, towing it is a royal pain in the a$$. Same trailer. 3800 engine in the van, the van has a lot more power for its size. The van is only slightly lower and the same width as the trailer. I have a hard time maintaining even 55 on the highway. 65 is out of the question. It feels like a constant hammering wind gust about every second that is slowing me down drastically at anything over 45. I wonder if the aero of the van has anything to do with it, maybe it is trying to reattach the airflow quicker than the 6000 wagon, then the trailer has to break it apart again.

Well, here are a couple 2008 Fullsize vans..

Ford

GM

Chrysler

- and even on your Pontiac minivan example, there's a good 2 feet in front of the windshield, which looks to me like it ends pretty much in-line with the front of the rim of the front tire. I'd guess the steering wheel is about in line with the side mirrors..


These vans... they're long gone.

http://www.mayswoodgarage.co.uk/ford..._van_small.jpg

:thumbup:

You could make the hood tilt forward.

Seeing that last van pic suddenly had me singing the scooby doo song:p ...in my cubicle...quietly..in my head.
A brick has better aerodynamics:turtle:

http://i122.photobucket.com/albums/o...CivicSI005.jpg

2003 Civic SI

I threw a teardrop into Flow Illustrator, going both forward and backward, with parameters Re=8000000 and dt=0.023.
Attachment 2885
Attachment 2886Attachment 2887

I think the ideal shape is neither the forward teardrop nor the reverse one. The objective here is to avoid a detached flow- the forward teardrop pushes a sizable wall of air, the reverse one allows turbulence to form at the end face, increasing drag.

The Ideal shape would be a combined teardrop with the front end formed by a reverse teardrop tail and the back end formed by a normal teardrop tail. The intermediate section can be tubular shape that then reaps benefit of a parting already taken place - something like the bullet trains of Shinkansen Japan or TGV France.

I'm pretty sure, dispite intuition, that for a given vehicle length and height, the teardrop gives the best cd.

Hi,

Here's the lowest Cd for a 4 wheeled vehicle:
http://www.spiegel.de/img/0,1020,1408871,00.jpg
http://img.photobucket.com/albums/v7...rd/pillbug.jpg
The Schlor Pillbug (1937) has a Cd of 0.13. Food for thought!

That's a sweet ride :)

Any of your aero folks want to punch that drag co-eff and a "standard" modern RR and weight into a hp calculator? I'd love to know how powerful of an electric motor that thing would require at 60mph.

A teardrop is the optimal shape.... for a teardrop. In reality, a car has different constraints than a drop of water. And a flat nose is NOT the same as a rounded blunt nose, so please stop saying all these cars would be more aerodynamic in reverse.

You can see that the pointy nose forward gives much better flow underneath the object, where it quickly attaches again. It will also have less pressure in front.

You can also see that having reduced slope behind is better for keeping the flow attached. Which leads to believe that the teardrop is NOT the ideal shape. Maybe something like this would be better:

Someone put that shape through flow illustrator, but use the same height and length as the teardrop.

hypermiler01 post 18 -- You're in my camp!

AFAIK, a pointy cigar is the optimal shape for a given volume with airflow parallel to the object. The only reason a rounded front is better is to be able to handle flow at an angle.

That's why the hulls of ships are cigar shaped with a pointy front and back.

Here is something interesting.. The optimal shapes for holding pressure in a submarine are cylinders and hemispheres. But that is not the best for drag, so submarines used to be designed with an external hull that wasn't sealed, and it would be pointy on both ends.

submarine: Definition from Answers.com

Ships (and some subs) need pointy bows for reasons other than drag. (Hint: think waves.)

Note that the part of a ship which works at the air/water boundary (ie. where there are waves) is generally pointy, but if it has a fully submerged drag reducing bulb, it is well, bulbous.

http://upload.wikimedia.org/wikipedi...ulbous_Bow.jpg

The purpose of the bulb is to change the shape of the bow wave so it doesn't interfere with hydrodynamics at the rear of the ship. It's optimized for a particular speed

Bulbous Bow

You can't confuse hydrodynamics and aerodynamics. They work pretty differently. We're not trying to pull your leg here. Simply google "lowest cd shape" and you'll find a million references to an airfoil (aka teardrop). Dig deeper and you'll see why the bulb is better than a flat front or a pointy front. Better yet, search around this site. This dead horse has been beaten into a unrecognisable bloody spatter.

I like to save this for special occasions:

http://spidermouse.us/images/smilies...iHitchcock.gif

:D

The only way that I can see how a round front is "better" than a pointy front are the other non-aerodynamic reasons for having a short overhang.

Those pictures Piwoslaw posted very clearly show better airflow under the nose. The pink stream quickly reattaches, and stays attached the entire length of the underside, while the pink line under the semi round front is destroyed and never reforms.

I would also bet that barometers placed at the nose would show better, lower readings in front of a pointy nose.

I am also inclined not to think that the engineers of Porsche, Ferrari, McClaren, Mercedes, Audi, etc. are completely clueless.

http://www.futurevehicletechnologies.../NewEvaro1.jpg
http://farm4.static.flickr.com/3031/...cee51f.jpg?v=0

http://www.speedace.info/speedace_im...skies_utah.jpg

And why do all the fast fish have pointy noses AND pointy tails?

http://www.scubaduba.com/robinson/shark_pilot.jpg

http://www.flmnh.ufl.edu/fish/Galler...ope_shark2.JPG

http://apgaylard.files.wordpress.com/2008/09/aptera.jpg

I mean seriously, don't those noobs over at Aptera realize that their design would be more efficient with a hemispherical nose? LOL

Well the lowest cD shape would be an infinitely long cylinder with 0 diameter, so any real world discussion of optimal shapes would involve length and volume constraints.

The way I see it is that the shallower the angle of the rear taper of the object, the lower the cD. But the front end also has to cut into the fluid and then allow smooth transition to parallel flow, and then stay attached to the rear taper.
If you have a constraint on length, sometimes the best thing to do is to get the rear taper as gradual as possible, which means less length available for the front end, so you end up with a hemisphere.

But if the hemisphere itself was the optimal shape for a front end, planes would be designed that way-- but they aren't. Their noses are shaped more like a half oval, or parabola. They are pointier than the theoretical teardrop.

In car design, you don't get to use as shallow a rear taper as physically possible, because you have to deal with design constraints. So you get it as shallow as you can, and then you design the front of the car based on the constraints created by mechanicals, windshield, driver, etc. There isn't much disadvantage to making the front pointy, because you'd only be adding a little more length. Compromising with a blunt front doesn't allow you to make the rear any shallower.

Since the pointy front would normally take the shape of a horizontal crease, it's not really affected by crosswinds.

Throwing my back woods logic into the mix... I would have a hard time arguing with mother nature as far as ideal shape at sub sonic speeds. Thereby making the water droplet moving through air the most aerodynamic shape under atmospheric pressure.

You're right, that was a bad example.

A better example would have been modern submarines designed primarily for fast, quiet underwater operation. They feature a rounded, not "pointy" bow.

But Matt is right: we need to be careful trying to comparing ordinary road vehicles to aircraft, watercraft or even speed record-setting land vehicles. The design rules change depending on the speed & size of the vehicle (in the same fluid), and between fluids of different densities.

Your basic mistake is assuming that the front of the water drop is aerodynamic. It isn't.

Surface tension of water pulls it into a perfect sphere. This has nothing to do with aerodynamics.

And the front of the water drop isn't pointy because those water molecules are being pressed back by air pressure in front, and pulled back by the surface tension.

If the front of the water drop were shaped like a high velocity bullet, it would fall faster.

Because the pointy nose does not cause as much of a pressure increase in the air in front of it.

So you ARE arguing with nature.

http://pileofphotos.com/pics/pic_1177083612.jpg

http://www.global-unity.com/CosmicDi.../07%20WIG5.gif

How so? Land speed cars are designed specifically for minimum air resistance at subsonic speeds, which is exactly what we want here.

The only reason that every car couldn't be shaped like a land-speed racer is because parking and speed bumps would be hell.

IHMO, the last LSR vehicle with an optimized cd was the Railton Special. A lot of engine guys just design for low frontal area and mostly teardrop shapes, and they are usually into downforce, too.

And nobody in the past 70 years has tried to optimize their cD for reduced air resistance? Even though the setting of records and winning of prizes depends on it?

http://ecomodder.com/forum/attachmen...0&d=1236641461

http://www.speedace.info/speedace_im...ad_drawing.jpg

John Cobb drew a nice shape and packed his machinery into it, and held the record far longer than anyone else. Mickey Thompson didn't even use a tapered tail, and missed the record by a technicality even after adding superchargers. The Summers Bros. used a more slender, tapered shape, with more HP and less frontal area to finally get Cobb's record. These days, traction, acceleration, and stability/downforce dominate LSR designs.
If you want to see how much engine men think about airflow, just try to find a streamlined roll bar. A bit of cardboard and tape can halve the drag on those, and proper fairings are far better than that.

Found a graphic illustrating the air pressure on the surface of a sphere traveling through air.

The reason that cars, fish, and planes have pointy noses is because they lower the air pressure in front, which causes less resistance to forward motion.

http://www.aerospaceweb.org/question...eal-sphere.jpg